The invention concerns a process for coating running webs of paper or cardboard where two coats are applied in direct succession, one upon the other, while the first coat is still moist. For both the first and second coatings there is at least one rotatable roll provided for support of the web, and at least the second coat is applied and dosed directly on the web.
A process for coating running webs with two successive coating operations, where the first coat is still moist as the second is applied, is known from the U.S. Pat. No. 3,202,536. The two coating stations are both arranged on a single roll guiding the web and on two successive rolls. A so-called airbrush is used as a final smoothing element. An applicator device of appropriate kind with a roll supporting the web on a coating device is known also from the Canadian Patent Document 882,640. In this case, the coat is applied by means of an applicator roll and scraped off by a doctor blade. A mixture for the coating substance is used that consists of a kaolin substance as well as starch, protein or thermoplastic synthetic resin. U.S. Pat. No. 3,202,536, provides that the first application coat has a relatively high consistency, i.e., with a solid substance content of more than 55%, while the second coat has a considerably lower consistency with a lower solid substance content of maximally 45%, where the major share is always kaolin.
Due to the high speeds in coating, using an airbrush is very problematic today; the airbrush can practically be used only up to speeds of 500 m/min. Considering the competitive pressures existing today, this can be a grave disadvantage, although the airbrush has the great advantage that with it a flawless coating can be achieved despite a relatively sparing application.
The problem underlying the invention is to propose a device and a process with which a flawless coating of a web can be achieved. This is true especially for use with cardboard, where the surface roughness of the base cardboard is relatively great, and where other coating processes, for instance such using a blade as equalizing element, may ensue bare spots at the protruding "mounds".